In the manufacture of multilayer ceramic (MLC) substrates for integrated circuit semiconductor package structures, metalizing paste is deposited in appropriate patterns and in the holes on the surfaces of the sheets, the sheets are stacked, and the sheets are subsequently fired at a sintering temperature. The package is made up of as many as five to one-hundred subcomponents (layers). Each of these layers represents a discrete level of wiring in the package. A layer is made up of two materials: ceramic and molybdenum paste, although other pastes, such as copper or tungsten, may be used. The ceramic forms the base or substrate and the molybdenum paste is used to personalize or "wire" the layer.
The wiring on each layer consists of interconnection holes known as vias and a distinct pattern on the layer surface. The vias are punched holes in the ceramic sheet that are filled with molybdenum paste. In a typical MLC product, the ceramic sheets have thousands of vias. The vias provide the interconnection to the next layer above or below. The pattern on the surface provides a vehicle to transmit signals in the x/y direction of the layer. Also the pattern connects to the layer above or below.
A partial cross-sectional view of a typical MLC is shown in FIG. 1. In FIG. 1, substrate 100 has a metalizing paste (not shown) disposed on a surface to form via 102 and lines 104. The cross-sectional dimensions of vias 102 and lines 104 are in the 0.100 mm to 0.200 mm (0.004 in. to 0.008 in.) range. It is of critical importance that the integrity of the lines and vias is maintained. An open/break in a line or via can result in an electrical defect and render the package unusable. Thus, it is extremely important that the paste is of a consistent high quality and is also air-free.
Air pockets of various sizes in the metalizing paste that is used to personalize ceramic greensheets cause defects. These air pockets cause two types of defects: voids in the lines on the greensheet surface or incomplete fills of the vias which interconnect the layers of the package. Therefore, it is necessary to de-aerate the metalizing paste before applying it to the MLC. The metalizing paste is highly viscous (thick) with a viscosity in the range of 35-60 Pa-sec (a measure of viscosity).
Each layer is personalized using a paste dispensing unit and a mask or stencil. A conventional dispensing unit is shown in FIGS. 2 and 3. FIG. 3 is a cross sectional view along line 3--3 of FIG. 2. As illustrated in FIGS. 2 and 3, dispensing unit 200 consists of a reservoir 300 to hold screening cartridge 310 containing paste 302 and a nozzle 304 for the delivery of paste 302 onto mask 202. Paste 302 is pressurized and flows from nozzle 304 into mask 202. The pressure of the paste 302 forces it into the openings in mask 202 and into any corresponding vias in substrate layer 308. The assembly is indexed along the mask (see arrow "A" in FIG. 2) in order to personalize the entire substrate layer. Holes (not shown) in mask 202 define the final pattern on the substrate layer and also ensure that all vias are filled.
The air pockets mentioned above originate during process steps in the fabrication of the paste as well as during the transfer of paste 302 to screening cartridge 310. Screening cartridge 310 allows paste 302 to be easily transferred from the large containers (typically used to contain the metalizing paste) to dispensing unit 200. Because the paste is extremely viscous, the act of removing air pockets becomes a challenging project. Attempting to draw the air out of the paste by conventional vacuum techniques proves to be a time consuming and inefficient process.
U.S. Pat. No. 4,987,852 issued to Sakai et al. discloses a conventional method for removing air from a low viscosity liquid and is illustrated in FIGS. 4A and 4B. In FIG. 4A, paint 400 (a low viscosity material) is introduced into upper section 402 of chamber 404 via inlets 406. Paint 400 flows in the direction of arrows "B". Paint 400 is drawn into lower section 408 of chamber 404 through strainer 410 by a vacuum supplied at vacuum port 412. Air released as paint 400 passes through strainer 410 is removed by vacuum port 412. To prevent paint 400 from being drawn into vacuum port 412, shield 414 protects vacuum port 412. The strained paint is removed from lower chamber 408 through lower port 420.
FIG. 4B is a view of strainer 410 through section 4B--4B of FIG. 4A. As shown in FIG. 4B, strainer 410 has slits 416 incorporated into the surface 418 of strainer 410. The passage of paint 400 through slits 416 results in ribbons of paint flowing from upper section 402 into lower section 408.
This approach has disadvantages in that highly viscous (thick) materials cannot be drawn into the lower chamber merely by vacuum and that creating slits in a strainer is very costly and time consuming.